The date range specified is based on the time zone selected under your Profile Settings. However, the system runs in the UTC timezone and as such the time period displayed on the graph axis will be in UTC. 

For example, stats for Dayparts between the hours of 6 am UTC +2 and 9 am UTC +2 will display in the graph on the Dashboard as 4 am UTC to 7 am UTC. Hovering over the graph will display the time in your time zone.